The present invention relates to camera system having a concurrent shutter (also referred to as global shutter) function, which is capable of preventing an occurrence of spurious signal even when an image of high-luminance object is taken.
MOS solid-state imaging devices are conventionally known as those using pixels having amplification/read function as solid-state imaging device. FIG. 1 shows a pixel construction of MOS solid-state imaging device. Shown respectively in FIG. 1 are: 100, a single pixel; 101, a photodiode serving as photoelectric conversion device; 102, a transfer transistor for transferring signal charge generated at photodiode 101 to a charge accumulation section (FD) 103; 104, a reset transistor for resetting the photodiode 101 and charge accumulation section 103; 105, an amplification transistor for amplifying and reading voltage level of the charge accumulation section 103; and 106, a select transistor for selecting the pixel so as to transmit an output of the amplification transistor 105 to a vertical signal line 114. These but photodiode 101 are shielded from light.
Further, denoted by 110 is a pixel power supply for applying a power supply voltage VDD, which is electrically connected to drain of the amplification transistor 105 and to drain of the reset transistor 104. Denoted by 111 is a reset line to which row reset signal φRSi for resetting pixels corresponding to one row is inputted, which is connected to the gate of reset transistor 104 of the pixels corresponding to one row. Denoted by 112 is a transfer line to which row transfer signal φTXi for transferring the signal charge of the pixels corresponding to one row to the charge accumulation section 103 of the respective pixel is applied, which is electrically connected respectively to the gate of transfer transistor 102 of the pixels corresponding to one row. Denoted by 113 is a select line to which row select signal φSELi for selecting pixels corresponding to one row is applied, which is electrically connected respectively to the gate of select transistor 106 of the pixels corresponding to one row. With the pixel construction using four transistors in this manner, a photoelectric conversion function, reset function, amplification/read function and temporary memory function are achieved.
The pixels having such construction are arranged into m-rows by n-columns to form a pixel array, and a normal XY-addressing read method (also referred to as rolling shutter read method) using a vertical and horizontal scanning circuits (not shown) is employed to sequentially select and read the pixel signals row by row from the first row to m-th row so as to read all pixel signals.
In such normal XY-addressing read method, however, the point in time for transferring/accumulating signal to/at the charge accumulation section 103 is different from one row to another of the pixel array. More specifically, there is a difference in point in time corresponding to one frame at maximum between the first row to be read out-first and m-th row to be read out at the end. For this reason, a distorted image problem occurs when a rapidly moving object is photographed.
The global shutter read method is a method for solving the above problem in the normal XY-addressing read method as described. An operation of the global shutter read method will now be described with reference to the timing chart shown in FIG. 2. First, as row reset signals φRS1 to φRSm and row transfer signals φTX1 to φTXm of all rows are simultaneously outputted from the vertical scanning circuit (not shown), photodiodes 101 of the pixels corresponding to all rows are reset. After that, subsequent to a certain signal accumulation period (exposure period), row transfer signals φTX1 to φTXm of all rows are simultaneously outputted from the vertical scanning circuit. The signal charges accumulated within the exposure period at photodiode 101 of the pixels corresponding to all rows are thereby transferred simultaneously for all rows to the electric charge accumulation section 103. With such operation, a global shutter operation is effected.
A row-by-row read of signal charges accumulated at the electric charge accumulation section 103 is then started. First, as row select signal φSEL1 is outputted, pixels of the first row are selected and signal levels of the pixels are read out. Further, as row reset signal φRS1 is outputted, the electric charge accumulation sections 103 of the pixels of the first row are reset, and the reset levels of the pixels are read out. When the readout of signal level and reset level of the pixels of the first row are complete, pixels of the second row are selected, and the signal level and reset level thereof are read out. By performing this signal read scanning until m-th row, signals of one frame are read out.
Further, Japanese Patent Application Laid-Open 2006-108889 has proposed a solid-state imaging device where a pixel array formed of 2m-rows by n-columns of single pixels having the pixel construction as shown in FIG. 1 is used to cause an image signal output as in the following. In particular, as shown in FIG. 3, a signal-to-be pixel group 200-1, 200-2, . . . where the signals of photodiode are simultaneously reset for all pixels and, after completion of a predetermined exposure period, the signals generated at photodiode are transferred to the charge accumulation section, and correcting pixel group 300-1, 300-2, . . . where the signals generated at photodiode are not transferred to the charge accumulation section are provided alternately on every other row in the pixel array so that difference between the respective signal outputs of the signal-to-be pixel group 200-1, 200-2, . . . , and of the correcting pixel group 300-1, 300-2, . . . is obtained and outputted as image signal.
FIG. 4 shows a timing chart for explaining operation of the solid-state imaging device having such construction. In FIG. 4, “1M-th to mM-th rows” refers to the first to m-th rows of the signal-to-be pixel group 200-1, 200-2, . . . , and “1S-th to mS-th rows” refers to the first to m-th rows of the correcting pixel group 300-1, 300-2, . . . , etc.
According to thus constructed solid-state imaging apparatus, if signals are sequentially read out row by row after concurrently transferring the signal charges to the electric charge accumulation section, signal retaining time at the charge accumulation section of the pixels of the rows which are read out late becomes relatively longer. While shading due to leak current or leakage light thereby tends to occur, an occurrence of such shading can presumably be prevented by the above described method where a difference signal is obtained.